In the case of chemical and biochemical reactions of substances in an aqueous solution, gas bubbles frequently form, which accumulate and rise to the surface of the solution. As this occurs, liquid and solid components of the solution are entrained therewith, which leads to the buildup of foam on the surface of the liquid. In the case of fermentation and/or digestion processes in bioreactors wherein broth is subjected to the action of microbes such as bacteria and yeast fungi, the accumulation of foam on the surface of the broth is particularly heavy. This foam accumulation in bioreactors occurs principally due to the evolution of carbon dioxide. Such a buildup of foam leads not only to the entrainment of liquid and solid components, thereby causing loss of a portion of the fermentation broth, but also effectively isolates the broth from atmospheric oxygen and blinds aeration filters, thereby interfering with or even stopping the fermentation process. To overcome these problems various schemes have been developed to prevent the formation of undesirable foam in a bioreactor or to repress, disperse or remove the foam.
For some time it has been known to mix in with the fermentation broth chemical antifoam agents to repress or diminish the formation of foam. Such antifoam agents, however, contaminate the fermenting broth, and must be removed later. Such agents can also negatively affect the reaction processes in the bioreactor and impair the exchange of biosubstances. In addition, due to the presence of fillers, thickeners, and coagulation agents in the antifoam agents, the agents tend to blind microfilters used in processing the broth, especially where membrane filters are employed.
To prevent such plugging of microfilters by antifoam agents, a process is disclosed in EP 0 391 590 A1, which uses an oil-based material encapsulated in a water-soluble granule antifoam agent. Although such an antifoam agent does reduce blinding of microfilters, it nevertheless still contaminates the fermentation broth.
DE 41 42 967 discloses another way of removing foam in bioreactors, by disseminating a liquid antifoam agent directly onto and/or into the foam itself. Although this method is effective at reducing foam buildup, it also still contaminates the broth.
Because of the foregoing drawbacks of antifoam agents, a number of attempts have been made to develop defoaming methods and apparatus which permit foam accumulation and then simply skim the foam off the broth, mechanically separate the foam""s solid and liquid components from its gaseous components and convey them into a collection container or back into the bioreactor to recombine with the original fermentation broth.
One such apparatus is described in DE 299 07 596 U1. The head space of the bioreactor is provided with a foam overflow line, which permits discharge of the foam into a collection container external to the bioreactor. The head space of this collection container is provided with a gas outlet open to the atmosphere. Foam that builds up within the head space of the bioreactor is gradually forced out and into the collection container. In the collection container the gaseous components separate from the liquid and solid components of the foam, and escape through the gas outlet. The liquid and the solid components of the foam concentrate in the bottom of the collection container and can be removed through an outlet line in the bottom of the collection container by a screw type pump. Because the foam virtually fills the entire head space of the bioreactor above the fermenting broth, the fermentation process is starved of oxygen and so slows, which must be compensated for by complicated, artificial aeration. Moreover, because of the unrestricted foam generation and escape of the foam from the bioreactor, an undesirable separation of a significant portion of the fermenting broth occurs, which brings about a substantial loss of liquid and cells which have been entrained in the foam.
It is therefore the goal of the present invention to provide a process and apparatus for the defoaming of the fermentation broth in a bioreactor which is effective, economical and space-saving, wherein the foam is promptly removed from the surface of the broth and the liquid and solid components of the foam are separated from the gaseous components and returned to the fermentation broth and wherein no contamination of the broth occurs.
The aforementioned goal is achieved by the provision of a foam guiding means, at least one skimmer plate rotating about an axis, and at least one centrifuge chamber, arranged together in the form of a rotor. The rotor is placed inside a bioreactor and above the broth surface, so that, by continuous rotation of the rotor, foam is mechanically skimmed off the broth surface and conducted into the centrifuge chamber, wherein the solid and liquid components of the foam are separated from gaseous components by centrifugal force and returned to the fermentation broth.
At lest two significant advantages are achieved by the inventive process and apparatus: (1) the addition of a chemical antifoam agent with its several known drawbacks becomes unnecessary; and (2) breakdown of the fermentation broth is avoided. The chemical reactions in the fermentation broth can proceed without interruption and any subsequent filtering of the broth is possible without the otherwise customary blinding of a microfilter. Since the rotor is positioned above the liquid level of the broth, and it rotates within the foam but not within the fermenting broth, a modest amount of energy is required to drive the rotor and thus, an economical defoaming operation is provided.